1. Field of the Invention
This invention relates to wireless communication systems, in particular, directional antennas for use in wireless communication systems.
2. Background
In wireless communication systems, antennas are used to transmit and receive radio frequency signals. In general, the antennas can be omni-directional, receiving and transmitting signals from any direction, or directional, with reception and transmission of signals limited in direction. In general, directional antennas provided increased gain over an omni-directional antenna because the directional antenna's coverage is focused over a small spatial region. Because a directional antenna covers a limited spatial region, the antenna needs to be “pointed” so that it can transmit and receive signals in a desired direction. Some conventional antenna systems include multiple directional antennas, or elements, arranged in an array such that individual elements “point” in different directions. By selecting desired elements of the array the overall direction of the antenna system can be varied. In addition, there exist antenna systems which provide directive gain with electronic scanning, such as phased arrays, rather than being fixed. However, many such electronic scanning technologies are plagued with excessive loss and high cost. In addition, many of today's wireless communication systems provide very little room for antennae elements.
One type of directional antenna that is popular is traditional Yagi-Uda (“Yagi”) antenna. A traditional Yagi antenna includes a driven element, the element a signal is fed to by a transmitter or other signal source, called the driver or antenna element, one or more reflectors, and one or more director elements. The reflector and director elements are parasitic elements that are not driven. By choosing the proper length and spacing of a reflector element from the driven element, as well as the length and spacing of director elements, the induced currents on the reflector and director elements will re-radiate a signal that will additively combine with the radiation from the driven element to form a more directive radiated beam compared to the radiation from the driven element alone. The most common Yagi arrays are fabricated using a dipole for the driven element, and straight wires for the reflector and director elements. The reflector element is placed “behind” the driven element and the director elements are placed in “front” of the driven element. The result is a linear array of wires that together radiate a beam of radio frequency (RF) energy in the forward direction. The directivity, and therefore the gain, of the radiated beam can be increased by adding additional director elements, but at the expense of overall antenna size. The director element can be eliminated, which leads to a smaller antenna with wider beam width coverage compared to Yagi antennas utilizing director elements.
In conventional Yagi antennas, the driven element is a dipole element that has a length that is nominally one-half of a wavelength of the radio frequency (RF) signal transmitted or received by the antenna. The reflector element is usually approximately five percent longer than the dipole and the director elements are approximately five percent shorter than the dipole. The spacing between the elements is critical to the design of the Yagi and varies from one design to another, with element spacing typically varying between one-eighth and one-quarter wavelength. While the Yagi antenna dos provide a relatively simple directional antenna design, the overall size is usually relatively large because of the reflector and director elements and the spacing between the elements.
There is a need in the art for improved antennas that can provide directional gain and are compact in size.